Co-stimulation of T cells leads to their expansion and activation. The CD28/B7 pathway is the central costimulatory pathway regulating immunity and autoimmunity. CD28 signaling is pivotal in the priming of pathogenic T cells that cause autoimmune diabetes. Two critical stimulatory ligands for CD28, B7-1 and B7- 2, are required for the priming, expansion, and demise of T cells during their life cycle. While these molecules behave similarly in some in vitro assays, they have divergent effects on the development of autoimmunity in the NOD mouse. When B7-1 is missing, NOD mice suffer from accelerated diabetes. However, when B7-2 is missing, NOD mice are completely protected from the disease. There have been a number of hypotheses put forth to explain these divergent observations. Indeed, if the actions of these costimulatory molecules could be understood, new therapies could be envisioned to treat autoimmune disease, without disturbing the entire T cell repertoire. The central goal of this application it to elucidate the mechanisms that explain the biological distinctions observed in the actions of B7-1 and B7-2. Three interlocking hypotheses will be tested. The initial set of experiments test the hypothesis that outside-in signaling into the ARC differentiates the actions of B7-1 and B7-2. This hypothesis is based upon differences between the structures and PKC phosphorylation sites of the cytoplasmic tails of B7-2 and B7-1. Specifically, we will determine whether differential PKC activation occurs in the absence of each chain. The second hypothesis is that B7-1 and B7-2 differentially affect the expansion and stability of bulk T cell populations in the mouse. This will be tested through BrdU incorporation studies and will allow us to determine the replication rate of T cell subcompartments in the absence of B7-1 or B7-2. Lastly, we hypothesize that B7-1 and B7-2 differentially regulate progression through the cell cycle in T cells. We propose to perform experiments to determine whether key pathways that regulate the cell cycle are perturbed in the absence of B7-1 and B7-2. We believe that the mechanistic information gained will address novel gaps in our knowledge regarding the relationship between co-stimulation and autoimmunity, allowing for more specific therapeutic intervention in the future.